Drive arrangement for activating a car safety device activation element

ABSTRACT

A drive arrangement for activating a car safety device activation element, such as an air bag, comprises a drive circuit, which is coupled to the car safety device activation element. The drive circuit generates an activation signal which activates the car safety device. The arrangement includes a power supply transistor which is coupled in series with a power supply input of the drive circuit and an energy reservoir such as a capacitor. The arrangement further comprises control means which controls the supply voltage to the drive circuit by controlling the power supply transistor to operate in an active region to provide a voltage drop during activation of the car safety device activation element. Hence, a significant voltage drop and thus energy dissipation may be moved from the drive circuit to the power supply transistor. The drive circuit may therefore be reduced in size and the power supply transistor may be implemented in a cheap technology suitable for energy dissipation.

This application is a continuation of U.S. patent application Ser. No.11/145,629, filed on Jun. 6, 2005, now U.S. Pat. No. 7,385,307, whichhas a common assignee, all of which is incorporated by reference in itsentirety.

FIELD OF THE INVENTION

The invention relates to an integrated circuit, a drive arrangement anda method for activating a car safety device activation element and inparticular, but not exclusively, to a drive arrangement for activatingan air bag activation element.

BACKGROUND OF THE INVENTION

In recent years car, car design has increasingly focused on safetyaspects including the performance of the car in crash situations.

In order to improve the safety of driver and passengers, modern carscomprise an increasing amount of safety devices. Many of these safetydevices are aimed at improving safety during crashes. One such safetydevice is an air bag which is activated during a crash to protect thedriver and passengers. Currently, cars typically comprise between oneand eight airbags and it is likely that this number will increase in thefuture.

It is of the outmost importance that safety devices such as air bags arereliably activated in the event of a crash. Furthermore, it is importantthat the air bags are only activated during a crash, as an unintendedactivation of an air bag may disturb a driver and possibly cause anaccident.

An air bag is typically activated by an activation element known as asquib. Different types of squibs exists but typically they are allactivated by a short pulse of significant energy. For example, one typeof squib comprises a very fast heating element which when applied thehigh energy pulse almost instantly generates a very high temperature.This ignites a small charge which sets of sodium azide resulting in thegeneration of a large volume of nitrogen gas filling the air bag.

In order to ensure a reliable air bag operation, it is critical that asuitable drive circuit is used for generating the activation pulse. FIG.1 illustrates a simplified air bag activation circuit in accordance withprior art.

FIG. 1 illustrates a squib 101 coupled to a drive circuit 103. The drivecircuit 103 is implemented in a single Application Specific IntegratedCircuit (ASIC) and comprises functionality for generating the activationpulse which activates the squib 101. More specifically, the drivecircuit 103 comprises a high side switch FET (Field Effect Transistor)105 and a low side switch FET 107. During normal operation, where theair bag is passive, the high side FET 105 and the low side FET 107 areboth in an off state and no current can flow through the squib. The useof two switch transistors in series provides increased reliability andfailure prevention. Particularly, if either one of the switch FETs shortcircuits, this will not result in an activation of the air bag as theother switch FET will be in the off state.

The high side FET 105 is controlled by a high side control circuit 109and the low side FET 107 is controlled by a low side control circuit111. The low side control circuit 111 produces a signal which switchesthe low side FET 107 off during normal operation and on if the air bagis being activated. The high side control circuit 109 also controls thehigh side FET 105 to be off during normal operation and on during airbag activation. However, rather than simply switching the high side FET105 fully on, the high side control circuit 109 also controls the signalto limit the current to the squib.

Typically, the current through the high side FET 105 is limited toaround 2 A. Typically, the same energy supply is used for a plurality ofair bags and the current limitation prevents that this energy supply isused up by a short circuit in one air bag. For example, during a crash,the upper squib end may be short circuited to ground. If the currentthrough the high side FET 105 is not limited, the resulting currentwould become exceedingly high thereby quickly draining the energy supplyand possibly preventing the activation of other air bags.

Typically, the drive circuit 103 is not directly connected to the energysupply. Rather, a power switch transistor known as a safing FET 113 iscoupled in series with the drive circuit 103. The safing switch 113 isgenerally an external discrete component. The safing FET 113 providesfurther failure prevention by providing additional redundancy in the airbag activation operation.

Specifically the operation of the safing FET 113 is controlled by acontrol circuit 115 in response to different detector inputs than usedfor activating the drive circuit. Typically the safing FET 113 iscontrolled by a completely different processing unit based in adifferent crash detection algorithm and sensor input than for the drivecircuit. Thus, the air bag is only activated if both redundantevaluations detect the occurrence of a crash in which case the high sideFET 105 and the low side FET 107 of the drive circuit as well as thesafing FET 113 are switched on. The safing FET 113 is operated as asimple on/off switch. In some applications several safing FETs are usedto provide independent safety switches for different drive circuits. Forexample, each air bag may be provided with its own safing FET.

The safing FET 113 is coupled to a reverse flow blocking diode 117. Itis an inherent feature of the manufacturing of FETs that a reverseparasitic diode 119, 121, 123 is connected between the source and drain.

The reverse flow blocking diode 117 is connected to a capacitor 125which provides the energy supply to the activation circuit. Thecapacitor 125 is mounted in close proximity to the air bag activationcircuit and ensures that energy may be provided to the air bagactivation circuit even if the connection to the battery is brokenduring the crash. However as the capacitor 125 may be discharged, forexample after the car has been switched of for a given duration, anelectrical path exists from the upper end of the squib to ground throughthe capacitor 125 and the parasitic diodes 119, 121.

Accordingly, in the absence of the blocking diode 117, a short circuitresulting in a voltage being applied to the lower end of the squib wouldresult in a current flowing through the squib and thereby activating theair bag. The blocking diode effectively breaks this path. The blockingdiode may typically be common to a plurality of drive circuits.

A number of disadvantages are associated with the prior art arrangementof FIG. 1.

Firstly, the requirement for an external safing FET tends to increasethe cost and complexity of the arrangement. Furthermore, the safing FETtends to be relatively bulky and as the FET is external to the drivecircuit, it requires additional operations during manufacturing.

Furthermore, the prior art design results in a significant energydissipation in the high side FET 105 which accordingly must berelatively large. Specifically, the energy stored in the reservoircapacitor is given by

$E = {\frac{1}{2}{C \cdot V^{2}}}$where C is the capacity of the capacitor and V is the voltage over thecapacitor. Hence, in order to store sufficient energy to ensure that thesquib is activated, while maintaining the size and cost of the capacitoracceptably low, it is required that the capacitor is charged to arelatively high voltage. Typically, the capacitor is charged to avoltage of around 35-36V.

During activation, the low side FET 107 is fully switched on resultingin a typical voltage drop of less than 2V. Furthermore, the impedance ofthe switch is relatively low resulting in a typical voltage drop of lessthan 2V. The voltage drop over the blocking diode 117 is typicallyaround 1V. Furthermore, the safing FET 113 is fully switched on duringactivation resulting in a typical voltage drop of around 1 V (the onresistance of the safing FET 113 is typically lower than that of the lowside FET 107). Accordingly, during the current limiting operation of thehigh side FET 105, the voltage drop from drain to source is typically inthe order of 30V. Typically the current is limited to around 2 A and thesquib is fired in typically 2 ms. Therefore, the energy dissipation inthe high side FET 105 during activation is around 30V·2 A·2 ms=120 mJ.This energy needs to be absorbed by the high side FET 105 withoutresulting in a thermal shutdown of the FET. In order to meet this energyrequirement, it is necessary that the high side FET 105 is physicallylarge.

However the requirement for a large FET has significant impact on theASIC cost. Furthermore, as the required size depends on the energyabsorption requirement, the design cannot take full advantage of theadvances in ASIC manufacturing technology. For example, as improvementsin lithography processing are achieved, smaller transistors can beformed resulting in smaller areas being required for circuits, Thisallows a higher integration and may allow more circuitry to be includedin the same ASIC.

Another disadvantage of the prior art arrangement of FIG. 1 is that theblocking diode 117 introduces a significant voltage drop. This voltagedrop results in an energy loss which must be compensated by an increasein the capacity of the capacitor. Furthermore, the blocking diode 117 istypically common for a plurality of air bags and thus carries a varylarge current during air bag activation. For example the blocking diode117 may be common for eight air bags thus conducting a typical currentof up to 16 A during a crash. Accordingly, the blocking diode 117 is arelatively large discrete component requiring additional operationsduring assembly of the arrangement.

Hence, an improved system for activating a car safety device would beadvantageous and in particular a system allowing for increasedflexibility, increased performance, increased integration, improvedreliability, reduced cost, reduced size and/or improved energyabsorption or dissipation would be advantageous.

STATEMENT OF INVENTION

The present invention provides a drive arrangement for activating a carsafety device activation element, an integrated circuit and a method ofactivating a car safety device as described in the accompanying claims.Accordingly, the present invention seeks to preferably mitigate,alleviate or eliminate one or more of the above-mentioned disadvantages,singly or in any combination.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention will now be described,with reference to the accompanying drawings, in which:

FIG. 1 illustrates a simplified air bag activation circuit in accordancewith prior art;

FIG. 2 illustrates a drive arrangement in accordance with an embodimentof the invention;

FIG. 3 illustrates an example of a voltage supply arrangement for aplurality of drive circuits in accordance with an embodiment of theinvention; and

FIG. 4 is an illustration of an integrated circuit comprising a drivecircuit and an supply voltage arrangement in accordance with anembodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The following description focuses on an embodiment of the inventionapplicable to an activation element for a car safety device whichspecifically is an air bag activation device (frequently referred to asa squib). However, it will be appreciated that the invention is notlimited to this application but may be applied to many other car safetydevices including for example a safety belt pretension activationelement. The described embodiment furthermore comprises Field EffectTransistors (FETs) but it will be appreciated that other transistortypes such as bipolar transistors may alternatively or additionally beused.

FIG. 2 illustrates a drive arrangement in accordance with an embodimentof the invention.

The drive arrangement is operable to generate an activation pulse whichmay activate the air bag activation element 201 henceforth referred toas the squib. The squib 201 is coupled to a drive circuit 203 comprisinga high side switch FET (Field Effect Transistor) 205 and a low sideswitch FET 207. The high side FET 205 is controlled by a high sidecontrol circuit 209 and the low side FET 207 is controlled by a low sidecontrol circuit 211.

During normal operation where the air bag is passive, the high side FET205 and the low side FET 207 are both in an off state. The low side FET207 may be operated and designed in the same way as for the prior artarrangement of FIG. 1. However, the high side FET 205 is designed anddimensioned differently and operated in a different way than for theprior art arrangement of FIG. 1 as will be described later. However,similarly to the circuit of FIG. 1, the drive circuit of the describedembodiment comprises current limiting means which limits the current forthe squib 201 during activation. In particular the high side FET 205 iscontrolled by the low side control circuit 211 to limit the currentthrough the high side FET 205 to around 2 A.

In the current embodiment, the drive circuit 203 is powered from anenergy reservoir in the form of a large capacitor 213. In otherembodiments, other energy supplies may used, such as for example abattery. In the described embodiment, the capacitor 213 is coupled tothe drive circuit 203 through a supply transistor 215, and in particularfor the current embodiment the supply transistor 215 is connected to thehigh side FET 205 in a series connection by the drain of the high sideFET 205 being connected to the source of the supply transistor 215. Inthe described embodiment, the supply transistor 215 is controlled by asupply controller 217. The supply transistor 215 is preferably, but notnecessarily, at least partly used as a safing transistor.

The supply transistor 215 is coupled to the capacitor 213 through ablocking FET 219. In particular, the drain of the supply transistor 215is coupled to the drain of the blocking FET 219 and the source of theblocking FET 219 is coupled to the capacitor 213.

During normal operation, all transistors of the drive arrangement areswitched off and no current flows through the squib 201. When a crash isdetected, the transistors switch to various on-states thereby allowingcurrent to flow through the squib resulting in this being activatedthereby causing the air bag to be activated. As an example, a typicalsquib may require an activation pulse of around 2 A for a duration ofaround 2 msec to be activated.

The operation of the drive arrangement during a squib activation will bedescribed in more detail in the following.

When safety circuitry (not shown) detects that a vehicle carrying theair bag is involved in a crash, it controls the drive arrangement toactivate the squib 201 and thus the air bag. The safety circuitry mayparticularly comprise one or more microcontrollers running suitablecrash detection algorithms based on suitable sensor inputs. When a crashis detected, the microcontroller preferably outputs a signal that causesthe low side control circuit 211 to generate a high gate voltage for thelow side FET 207. The gate voltage is preferably sufficiently high todrive the low side FET 207 into the non-saturated operation region. Inthe non-saturated operating region, a FET operates similarly to aresistor having a low value, R_(DS,ON), which is dependent on thevoltage between the gate and source. Hence, the low side FET 207 ispreferably driven to provide a very low substantially resistive load.The FET may specifically be considered to be in the non-saturatedoperation region when|V _(DS) |<|V _(GS) −V _(T)|where V_(DS) is the drain-source voltage, V_(GS) is the gate-sourcevoltage and V_(T) is the threshold voltage of the transistor.

Similarly, the blocking FET 219 is switched on by preferably providing asufficiently high gate voltage to drive it into the non-saturatedregion.

The safety circuitry also generates a signal causing the high sidecontrol circuit 209 to switch on the high side FET 205. However, incontrast to the low side FET 207 and the blocking FET 219, the high sideFET 205 is not (necessarily) driven into the non-saturated region.Rather the high side FET 205 is driven in a current limitation modewherein the current through the high side FET 205 and thus the squib 201is limited to a suitable value which typically may be around 2 A.Typically, this will result in the high side FET 205 operating in anactive operating range. For a FET, the active operating range may bedefined as the region for which|V _(DS) |≧|V _(GS) −V _(T)|where V_(DS) is the drain-source voltage, V_(GS) is the gate-sourcevoltage and V_(T) is the threshold voltage for the transistor. For a FETthe active operating range may sometimes be referred to as the saturatedregion or the linear region.

Thus, in contrast to the low side FET 207 and the blocking FET 219, thehigh side FET 205 is not fully switched on but is rather controlled bythe high side control circuit 209 to provide a suitable current for thesquib 201. It will be appreciated that any suitable method of measuring,estimating or determining the current through the squib 201 may be used.For example, the high side control circuit 209 may sense the voltagedrop over a small resistor in series with the squib 201 as will be wellknown to a person skilled in the art.

Similarly, when the safety circuitry indicates that a crash is ongoing,the supply controller 217 switches on the supply FET 215 to allowcurrent to flow through the transistor. However, in contrast to thecircuit of FIG. 1, the supply controller 217 controls the supply FET 215to operate in the active region such as to provide a voltage drop duringactivation of the car safety device activation element. Thus, the supplyvoltage for the drive circuit 203 is controlled by the supply controller217. Hence, in contrast to the circuit of FIG. 1, the supply FET 215 isnot fully switched on to provide a low resistance with an insignificantvoltage drop but is rather driven into the active region whereby asubstantial voltage drop over the supply FET 215 is achieved.

In a simple embodiment, the supply FET 215 may be controlled to providea substantially constant supply voltage for the drive circuit. In asimple embodiment, the supply controller 217 may for example set thegate voltage of the supply FET 215 to a substantially constant value.For example, if a gate source voltage of 3V corresponds to a drainsource current of around 2 A in the active region, the gate voltage maybe set at a fixed level of 15V resulting in a source voltage of around12V. Thus a significant voltage drop occurs from drain to source of thesupply FET 215 rather than over the high side FET 205.

In the described embodiment, the supply FET 215 is thus controlled toreduce the voltage at the input to the drive circuit. Accordingly, thepower absorption in the drive circuit is substantially reduced incomparison to the prior art arrangement of FIG. 1. In the specificexample, assuming the voltage drop over the squib 201 is 2V and thevoltage drop over the low side FET 207 is 2V, the voltage drop acrossthe high side FET 205 is reduced from around 32V (ignoring the voltagedrop of any blocking components) to around 8V corresponding to areduction of the energy to be dissipated from 128 mJ to 32 mJ (assumingan activation pulse of 2 A for 2 msec and negligible capacitor voltagechange). Hence, in the embodiment, the energy dissipation of the highside FET 205 is reduced by a factor of four.

The drive circuit is in the described embodiment implemented in a singleASIC. For the high side FET 205, the physical dimension is driven by therequired capacity for absorbing the generated heat energy during theactivation without resulting in a thermal shutdown. A thermal shutdowntypically occurs at a temperature of around 300° C.-400° C. For silicontechnologies, the shutdown temperature is relatively independent of thetechnology used and the smaller dimensions of more advanced technologiescan therefore not be fully exploited by the design.

Furthermore, as the high side FET 205 is the component of the drivecircuit 203 which typically generates the largest heat energy, the sizeof the high side FET 205 is typically a limiting factor when designingthe ASIC. Accordingly, a reduction of the energy dissipation of the highside FET 205 provides substantial advantages and may in particularprovide for a higher integration. For example, the significant sizereduction may result in significant amounts of additional circuitrybeing included in the same ASIC thus providing for increasedfunctionality and reduced overall cost. Furthermore, the reduced powerdissipation may increase the overall reliability of the drive circuit203.

As an example, the length of the circumference of the high side FET 205will approximately be proportional to the required energy absorption.Therefore, a reduction of the dissipated energy by a power of four willresult in the size of the high side FET 205 being reduced by a largedegree and the reduction in area may even exceed the reduction indissipated energy. In a typical conventional drive circuit, the highside FET 205 may account for around 60% of the total semiconductor area.Reducing this by a factor of four results in only 15% of the area beingtaken up by the high side FET 205 leaving an additional 45% foradditional circuitry. In this example, the area available for e.g.various control circuitry is more than doubled thereby allowing for theASIC to potentially comprise more than twice the functionality of aconventional ASIC.

In comparison to the circuit of FIG. 1, the control of the supply FET215 to provide a significant voltage drop by being operated in theactive region results in a significant shift of the power dissipationfrom the high side FET 205 to the supply FET 215. Accordingly, the powerrequirements for the supply FET 215 will typically be stricter for thecurrent embodiment than for the circuit of FIG. 1. However, the supplyFET 215 is preferably implemented in a less critical technology wherethe increased power requirement is of less significance. Specifically,for a discrete FET, the increased power requirement may easily be takeninto account and can typically be met by the same FET as used in thecircuit of FIG. 1.

In the described example, the voltage drop over the supply FET 215 wasaround two thirds of the capacitor voltage when fully charged. It willbe appreciated that the circuit may be designed to provide any suitablevoltage drop over the supply FET 215. Preferably, the voltage drop issuch that during the activation pulse, the energy dissipated in thesupply FET 215 exceeds that dissipated in the drive circuit and inparticular in the high side FET 205. Thus, preferably the majority ofthe energy is dissipated in the supply FET 215 rather than in the highside FET 205.

As the capacitor is discharged, the voltage over of the capacityreduces. However, the supply voltage may remain relatively constantuntil a stage where the supply FET 215 enters the non-saturatedresistive operating region. Preferably the voltage drop is significantat the initiation of the activation. Specifically, the voltage drop ofthe supply FET 215 preferably exceeds at least half the supply voltageof the capacitor in order to provide a substantial dissipation of energyin the supply FET 215 rather than in the high side FET 205.

Preferably, the design is such that the supply FET 215 remains in theactive region at least until the squib 201 fires. However, in someembodiments, the voltage drop over the supply FET 215 is not significantfor the whole duration of the activation pulse. However, in order toallow for a significant energy dissipation, the voltage drop preferablyexceeds half the supply voltage of the drive circuit 203 for more than aquarter of a time interval from initiation of the activation signal toan activation of the drive circuit occurs. In particular, the voltagedrop is preferably at least 5V for a duration of at least 0.5 msec ofthe activation pulse.

Preferably the supply FET 215 not only provides the regulation of thesupply voltage to the drive circuit 203 but also acts as a safingswitch. In the described embodiment, separate redundant circuits areused for the drive circuit and the supply FET 215 in determining if anactivation of the air bag is required. Specifically, the supply FET 215is controlled by a separate microcontroller to that controlling theactivation of the drive circuit 203. The separate microcontrollerexecutes separate and preferably different crash detection algorithmspreferably based on separate sensor inputs. The air bag is fired whenboth microcontrollers independently determines that a crash detectionoccurs. The safing switch thereby provides additional redundancyeffectively preventing that a single point failure results in anerroneous activation of the air bag.

In the described embodiment, a single transistor is thus used for twodifferent purposes namely for regulating (specifically reducing) asupply voltage to the drive circuit and for providing additional failuremitigation. The complexity of the circuit is therefore kept low and inparticular no complexity increase is required with respect to thecircuit of FIG. 1.

In some embodiments, the combined safing and supply FET 215 is coupledto an external connection allowing drive circuits to be connected to it.This provides enhanced design freedom for the board design and allowshigh flexibility and in particular allows any suitable number of drivecircuits to be connected to the same external connection thereby sharingthe same safing and supply FET 215.

It will be appreciated that the control of the supply voltage of thedrive circuit may correspond to providing a more stable and less varyingsupply voltage but that this is not essential. Rather in someembodiments it may be desirable to dynamically control the voltage dropover the supply FET 215 during the activation of the squib 201. Thecontrol may be an active control in order to provide a desired supplyvoltage variation during the activation or may be a passive controlwhere the supply voltage varies due to the variation of otherparameters, such as for example the voltage variation over the capacitoras this discharges.

In particular, the supply FET 215 may be controlled in response to theactivation current of the squib 201. For example, the squib 201 currentmay be measured and if this falls below a given threshold, the supplyvoltage may be increased to enable a higher current through the highside FET 205.

In the embodiment of FIG. 2, the supply FET 215 is coupled to thecapacitor 213 through the blocking FET 219. The blocking FET 219 ismounted in a reverse configuration to the supply FET 215 such that thedrain of the supply FET 215 is connected to the drain of the blockingFET 219. Specifically, in the specific embodiment, the blocking FET 219is implemented on the same semiconductor as the supply FET 215 with thetwo FETs having a common drain. Accordingly, the parasitic diode 221 ofthe blocking FET 219 is in the opposite direction of the parasitic diode223 of the supply FET 215. Therefore, the parasitic diode 221 preventsany current flowing through the parasitic diode 223 of the supply FET215. In other words, the parasitic diode 221 provides the same blockingeffect as the blocking diode 117 of the circuit of FIG. 1.

However, during an activation of the air bag, the activation current isnot carried through the parasitic diode 221. Rather, the blocking FET219 is switched fully on and the activation current is conducted throughthe FET rather than the parasitic diode 221. Specifically, the gate ofthe blocking FET 219 may be fed the capacitor voltage thereby drivingthe blocking FET 219 into the non-saturated operating mode. In thismode, the blocking FET 219 presents a very low resistive value resultingin a reduced voltage drop.

For example, assuming that a combined current of 10 A must be conductedduring activation of multiple air bags. A blocking diode carrying thislarge current will typically have a large voltage drop of typicallyaround 1.2V. However, the effective resistance of the blocking FET 219in the non-saturated mode may be 50 mΩ resulting in a total voltage dropof only 0.50V.

The reduced voltage drop over the blocking component may reduce thepower consumption and dissipation. Furthermore, it may allow a reducedcapacitance of the capacitor. Since the energy stored in the capacitoris proportional to the square of the charged voltage, even a relativelysmall reduction in the voltage may result in a significant increase ofthe stored energy thus reducing the requirement for the capacitanceresulting in a significantly smaller and cheaper capacitor.

Thus, using the blocking FET 219 in the non-saturated (R_(DS,ON)) modemay result in a reduced voltage drop which is particularly advantageouswhen the capacitor voltage becomes relatively low (such as when it fallsto around 8-9V). However, it will be appreciated that it is notessential to conduct the current through the blocking FET 219 in anon-saturated mode but that for example an opened diode configurationmay be used instead.

Furthermore, the blocking FET 219 may typically be simpler to implementthan the blocking diode 117 of FIG. 1. Preferably, the blocking FET 219and the supply FET 215 are integrally formed on the same semiconductorsubstrate with a shared drain. Thus, the combined functionality of theblocking FET 219 and the supply FET 215 may be achieved with littleadditional cost over that of each of the FETs individually.

The blocking FET 219 is preferably common for a plurality of supplyFETs. FIG. 3 illustrates an example of a voltage supply arrangement fora plurality of drive circuits in accordance with an embodiment of theinvention. In the example, a blocking FET 301 has common drain withthree supply FETs 303, 305, 307. The four FETs are implemented on thesame semiconductor substrate. The parasitic diode 309 of blocking FET301 provides a blocking effect in the same way as described above withreference to FIG. 2.

In operation, the source of the blocking FET 301 is coupled to theenergy reservoir such as a capacitor. The source of each individualsupply FET 303, 305, 307 is coupled to a drive circuit for a car safetydevice activation element. For example, the source of the first supplyFET 303 may be coupled to a drive circuit for a driver air bag, thesource of the second supply FET 305 may be coupled to a drive circuitfor a passenger air bag, and the source of the third supply FET 305 maybe coupled to a drive circuit for a seat belt pretensioner. The gate ofeach of the blocking FET 301 and supply FETs 303, 305, 305 are attachedto connectors for connection to suitable control circuits. Thus each ofthe supply FETs 303, 305, 305 may be individually controlled to providepreferably both a voltage regulating and a safing switch function.Hence, a simple, low cost and efficient arrangement for providing asupply voltage for a drive circuit may be provided. Furthermore, as thecomponents may be integrated on the same semiconductor substrate, anoverall size reduction may be achieved and the need for separate(possibly discrete) components is avoided.

The individual supply FETs 303, 305, 307 may be dimensioned to match therequired performance for each drive circuit and/or car safety device.For example, the squibs activated by the first and second supply FETs303, 305 may require twice the current of the seat belt pretensionersquib activated by the supply FET 307. Accordingly, the first and secondsupply FETs 303, 305 may be made twice as big as the third supply FET307.

Preferably the supply FET 215 and/or the blocking FET 219 is integratedin the same package as the drive circuit 201. In particular, an ASICcomprising the drive circuit 203 also comprises a supply voltagearrangement comprising the supply FET 215 and preferably the blockingFET 219. The supply voltage arrangement may comprise additionalcomponents such as supply FETs for other drive circuits (which may alsobe comprised in the ASIC). Specifically, the supply voltage arrangementmay correspond to the supply voltage arrangement illustrated in FIG. 3.

In the following, an embodiment of an integrated circuit comprising botha drive circuit and a supply voltage arrangement will be described. Inthe embodiment, the drive circuit and the supply voltage arrangement areimplemented on separate dies within the same package.

FIG. 4 is an illustration of an integrated circuit comprising a drivecircuit and an supply voltage arrangement in accordance with anembodiment of the invention.

The ASIC 400 comprises a first die 401 which implements thefunctionality of the drive circuit. The die may further comprise othercircuitry such as control circuitry e.g. for controlling the drivecircuit. In addition the ASIC 400 comprises a second die 403 whichimplements the supply voltage arrangement.

An advantage of implementing the drive circuit and the supply voltagearrangement on separate dies is that different technologies can be usedin implementing each circuit. Thus, the first die may be optimised forthe requirements and characteristics of the drive circuit whereas thesecond die may be optimised for the requirements and characteristics ofthe supply voltage arrangement.

Specifically, the first die may utilise a more advanced technology thanthe second die. The first die may use a technology which is relativelyexpensive to manufacture but which allows low dimensions and thus highintegration. For example, the first die may be implemented using aMotorola™ SMARTMOS™ technology such as SMOS7MV which allows designdimensions of around 0.25 μm.

In contrast, the second die is preferably implemented using a simplerand cheaper technology such as a vertical MOSFET technology like forexample an HDTMOS technology.

Thus, the advantages of the SMARTMOS™ technology may be used to providehigh integration and additional functionality whereas the cheaper HDTMOStechnology may be used for the supply voltage arrangement which cannottake advantage of the potential lower dimensions due to the energydissipation requirement. Hence, in the ASIC, the majority of energydissipation is accomplished by the cheapest and most robust technologywhereas the logic and control circuitry is implemented in a moreadvanced technology.

In the ASIC 400 of FIG. 4, the first and second dies 401, 403 aremounted on a common support element 405. Preferably the first and seconddie 401, 403 are electrically isolated from each other. Specifically, inthe arrangement of FIG. 4, it can be seen that the common drain, whichis typically formed by the substrate of the second die, is at a highvoltage potential during activation. Specifically, the voltage potentialmay be around 35V at the outset of the activation pulse.

However, the substrate of the first die in which the drive circuit andlogic circuits are formed is maintained at a ground level. Therefore,the substrates must be isolated from each other in this embodiment.

The isolation may in some embodiments be achieved by the sue of a commonsupport element which includes isolation means between the first die andthe second die. In particular, the common support element may itself beof an isolating material, and the first and second die may be adhered tothis common support element.

Alternatively or additionally, the first and/or second die may beattached to the common support element using isolating glue. This may beparticularly suitable for embodiments where the potential voltagedifferential between the first and second die is relatively low. In onesuch embodiment, the common support element may be formed by thesubstrate of the first (or second) die and the second (or first) die maybe glued onto the substrate of the first (or second) die using isolatingglue.

In some embodiments, different car safety devices may be fed bydifferent energy supplies. In particular, some safety devices, such asair bags, may be fed from a locally placed capacitor whereas other lesscritical safety devices, such as seat belt pre-tensioners, may be fedfrom the car battery. In this case, the available voltage levels fordifferent safety devices may vary significantly. For example, thecapacitor may be charged to a voltage of 35V whereas the car batteryvoltage may be in the range from 14V to 18V. In this case, drivecircuits used with the capacitor energy resource are preferably fedthrough a blocking FET and a supply FET as previously described.However, drive circuits fed from the battery may be fed directly due tothe reduced voltage levels of the battery. Accordingly, the drivecircuit of an ASIC is preferably designed to operate either directlyconnected to a battery or coupled to a capacitor through a supplyvoltage arrangement as previously described.

As the battery voltage may be slightly higher than the regulated voltagefrom the supply voltage arrangement, this may require that the high sideFET of the drive circuit is increased in size but this may be anacceptable trade-off in many embodiments in view of the enhancedcompatibility of the drive circuit for different applications. Thus thesame ASIC may be used both for capacitor driven and battery drivensafety devices.

The above description has referred to an active region and anon-saturated region of a FET. It will be appreciated that thecorresponding regions of a bipolar transistor may be equally applicable.For example, the non-saturated region of a FET may correspond to thesaturated region of a bipolar transistor. Similar, the active region forthe FET may correspond to the active or normal operating region for abipolar transistor.

The invention, or at least embodiments thereof, tend to provide one ormore following advantages, singly or in combination:

-   -   (i) The energy dissipation of a car safety device drive        arrangement may be moved to cheaper, more robust technology        and/or to external components.    -   (ii) The number of external components may be reduced.    -   (iii) An improved blocking arrangement with reduced power        dissipation during activation of the car safety device may be        achieved.    -   (iv) A higher integration may be achieved.    -   (v) The cost of the car safety device drive arrangement may be        reduced.

The invention can be implemented in any suitable form. The elements andcomponents of an embodiment of the invention may be physically,functionally and logically implemented in any suitable way. Indeed thefunctionality may be implemented in a single unit, in a plurality ofunits or as part of other functional units.

Whilst the specific and preferred implementations of the embodiments ofthe present invention are described above, it is clear that one skilledin the art could readily apply variations and modifications of suchinventive concepts.

Thus, the scope of the present invention is limited only by theaccompanying claims. In the claims, the term comprising does not excludethe presence of other elements or steps.

1. A drive arrangement for activating a car safety device activationelement, the drive arrangement comprising: a drive circuit for couplingto the car safety device activation element and operable to generate anactivation signal for activating the car safety device activationelement; a power supply transistor coupled in series with a power supplyinput of the drive circuit; a controller for controlling a supplyvoltage for the drive circuit by controlling the power supply transistorto operate in an active region to provide a voltage drop duringactivation of the car safety device activation element; and a reverseblocking switch transistor in series with the power supply transistor.2. The drive arrangement according to claim 1, wherein the drive circuitcomprises current limiting device limiting a current for the car safetydevice activation element during activation of the car safety deviceactivation element.
 3. The drive arrangement according to claim 1,wherein the drive circuit comprises a first drive transistor in serieswith the power supply input and a first output for coupling to the carsafety device activation element.
 4. The drive arrangement according toclaim 3, wherein the drive circuit comprises a second drive transistorin series with a second output for coupling to the car safety deviceactivation element.
 5. The drive arrangement according to claim 1,wherein the controller is operable to control the power supplytransistor such that the voltage drop exceeds half the supply voltagefor more than a quarter of a time interval from initiation of theactivation signal to an activation of the car safety device activationelement.
 6. The drive arrangement according to claim 1, wherein thecontroller is operable to control the power supply transistor such thatan energy dissipated in the power supply transistor during activationexceeds an energy dissipated in the drive circuit during activation. 7.The drive arrangement according to claim 1, wherein the controller isoperable to dynamically control the voltage drop during activation ofthe car safety device activation element.
 8. The drive arrangementaccording to claim 1, wherein the controller is operable to control thevoltage drop in response to an activation current of the car safetydevice activation element.
 9. The drive arrangement according to claim1, wherein the reverse blocking switch transistor is common to aplurality of safing transistors.
 10. An integrated circuit comprising adrive arrangement for activating a car safety device activation element,the drive arrangement comprising: a drive circuit for coupling to thecar safety device activation element and operable to generate anactivation signal for activating the car safety device activationelement; a power supply transistor coupled in series with a power supplyinput of the drive circuit; a controller for controlling a supplyvoltage for the drive circuit by controlling the power supply transistorto operate in an active region to provide a voltage drop duringactivation of the car safety device activation element; and a reverseblocking switch transistor in series with the power supply transistor.11. The integrated circuit according to claim 10, wherein the powersupply transistor is disposed on a first die and the drive circuit isdisposed on a second die.
 12. The integrated circuit according to claim11, wherein the first die and the second die are mounted on a commonsupport element.
 13. The integrated circuit according to claim 12,wherein the common support element includes isolation means between thefirst die and the second die.
 14. The integrated circuit according toclaim 12, wherein at least one of the first die and the second die isattached to the common support element by isolating glue.
 15. Theintegrated circuit according to claim 11, wherein the first diecomprises a plurality of safing transistors.
 16. The integrated circuitaccording to claim 15, wherein the plurality of safing transistors havea common drain and a separate source for coupling to different carsafety device activation elements.
 17. The integrated circuit accordingto claim 11, wherein the first die comprises a reverse blocking switchtransistor.
 18. The integrated circuit according to claim 17, wherein adrain of the reverse blocking switch transistor is connected to a commondrain of a plurality of safing transistors.
 19. The integrated circuitaccording to claim 10, wherein the power supply transistor is formed inaccordance with a first semiconductor technology and the drive circuitis at least partly formed in accordance with a different semiconductortechnology.
 20. A method of activating a car safety device activationelement using a drive arrangement comprising a drive circuit forcoupling to the car safety device activation element and operable togenerate an activation signal for activating the car safety deviceactivation element, a power supply transistor coupled in series with apower supply input of the drive circuit, and a reverse blocking switchtransistor in series with the power supply transistor, the methodcomprising the step of: controlling a supply voltage for the drivecircuit by controlling the power supply transistor to operate in anactive region to provide a voltage drop during activation of the carsafety device activation element, wherein the energy dissipated in thepower supply transistor during activation exceeds an energy dissipatedin the drive circuit during activation.